In recent years there has been rapid progress in wireless transmission technology, and in particular there have been dramatic improvements in data transmission speeds.
Until several years ago, the 64 kbps of PHS (Personal Handy-phone System) was the highest transmission rate. At present, the 2.4 GHz band IEEE 802.11b specification enables transmission speeds of up to 11 Mbps. And the 5 GHz band IEEE 802.11a and MMAC (Multimedia Mobile Access Communication System), HiSWANa (High Speed Wireless Access Network Type a), and BRAN (Broadband Radio Access Networks) HiperLAN/2, transmission speeds of 20 to 30 Mbps and above are realized. When transmission speeds reach such levels, wireless realtime transmission of a plurality of video data streams, encoded using MPEG (Moving Picture Expert Group) 1/MPEG2/MPEG4 or other video encoding methods, becomes possible. The above IEEE 802.11a, HiSWANa, and HiperLAN/2 use a modulation method called OFDM (Orthogonal Frequency Division Multiplexing). Keeping with user requests, this modulation method can be time-division multiplexing of channels using modulation methods with high transmission quality but low transmission speed and channels using modulation methods with poor transmission quality but high transmission speed.
The transmission quality is a parameter used to evaluate performance in transmission and reproduction of data; often the BER (bit error rate) or other values are used.
One technique to suppress the reduction of image quality in wireless transmission of images encoded using a video encoding method, of which MPEG1, MPEG2 and MPEG4 are representative, is an “error correction encoding device and decoding device” described in Japanese Patent Laid-open No. 11-330984. In the above publication, when transmitting an encoded image stream over a wireless transmission channel using a modulation method such as OFDM, the stream is divided into important information A and non-important information B. The important information portion is transmitted using modulation method α, with high transmission quality and low transmission speed. The non-important information is transmitted using modulation method β, with low transmission quality but high transmission speed. In this way, a comparatively high image transmission rate can be maintained while preventing large disruptions of image quality.
However, the above-described background art has the following problem. In general, when encoding video data, there are a variable bit rate (VBR), in which the timing of the bit rate (transmission speed) of the output bitstream changes with the content of the video data, and a constant bit rate (CBR), which is controlled such that the bit rate is constant. These are used selectively according to the application. For realtime transmission over networks, CBR, with a constant transmission speed, is suitable.
In a modulation method such as OFDM, in order to perform transmission using two modulation methods with different transmission speeds, even when using a method such as CBR with a constant transmission speed, if the proportion of the number of bits per unit time of the above important information A and non-important information B changes, the transmission speed may change at any time.
FIG. 13A and FIG. 13B are figures showing details of transmission speeds. The total number of bits per unit time for a CBR bitstream is essentially constant even if the proportion of important information A and non-important information B changes, as shown in FIG. 13A. However, this excludes errors occurring in the algorithm for bit rate control. When transmitting this information using a single modulation method, even if the proportion between A and B changes, as in (1) and (2), if a fixed amount of time can be secured, transmission without problems is possible. However, as shown in FIG. 13B, if the bitstream of important information A is modulated using modulation method α and the non-important information B is modulated using modulation method β for transmission over a wireless channel, if the proportion per unit time of the important information A and non-important information B changes as in (1) and (2), because of differences in the amount of transmission per unit time for modulation method α and modulation method β, the transmission time changes. That is, the total number of bits transmitted per unit time (the transmission speed) changes.
If, as explained above, the transmission time changes frequently, the time required for a bitstream increases. Hence it may occur that the increased amount of time cannot be secured, so that delays and losses occur in the bitstream transmission, causing significant degradation of the reproduced video. It may also happen that there is insufficient transmission time for another communication application. It is also possible to respond one-by-one to requests from the video encoding means, so as to dynamically secure transmission time; however, this complicates the system and increases the load on the wireless transmission/reception device.
An object of this invention is to effectively utilize bandwidth through transmission in a fixed amount of time, even when a plurality of modulation methods with different transmission rates are used to transmit a video bitstream.